Conventional electrochemical cells, such as alkaline cells, are formed of a cylindrical housing having an open end and an end cap assembly inserted therein to seal the housing. Conventional alkaline cells typically comprise an anode comprising zinc, a cathode comprising manganese dioxide, and an alkaline electrolyte comprising aqueous potassium hydroxide. After the cell contents are supplied, the cell is closed by crimping the housing edge over the end cap assembly to provide a tight seal for the cell. The end cap assembly comprises an exposed end cap plate which functions as a cell terminal and typically a plastic insulating plug, which seals the open end of the cell housing. A problem associated with design of various electrochemical cells, particularly alkaline cells, is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally near the point of complete exhaustion of the cell's useful capacity. Electrochemical cells, particularly alkaline cells, are conventionally provided with rupturable diaphragms or rupturable membranes within an end cap assembly. The rupturable diaphragm or membrane may be formed within a plastic insulating member as described, for example, in U.S. Pat. No. 3,617,386. Such diaphragms are designed to rupture when gas pressure within the cell exceeds a predetermined level. The end cap assembly may be provided with vent holes for the gas to escape when the diaphragm or membrane is ruptured. The end cap assembly disclosed in U.S. Pat. No. 3,617,386 discloses a grooved rupturable seal diaphragm and a separate metal contact disk between the end cap and seal diaphragm. The end cap assembly disclosed in the reference is not designed to withstand radial compressive forces and will tend to leak when the cell is subjected to extremes in hot and cold climate.
In order to provide a tight seal contemporary prior art typically disclose end cap assemblies which include a metal support disk inserted between the end cap plate and an insulating member. The separate metal support disk is radially compressed when the cell housing edge is crimped over the end cap assembly. The insulating plug is typically in the form of a plastic insulating disk which extends from the center of the cell towards the cell housing and electrically insulates the metal support disk from the cell housing. The metal support disk may have a highly convoluted surface as shown in U.S. Pat. Nos. 5,759,713 or 5,080,985 which assures that end cap assembly can withstand high radial compressive forces during crimping of the cell's housing edge around the end cap assembly. This results in a tight mechanical seal around the end cap assembly at all times.
The prior art discloses rupturable vent membranes which are integrally formed as thinned areas within the insulating disk included within the end cap assembly. Such vent membranes can be oriented such that they lie in a plane perpendicular to the cell's longitudinal axis, for example, as shown in U.S. Pat. No. 5,589,293, or they may be oriented so that they are slanted in relation to the cell's longitudinal axis as shown in U.S. Pat. No. 4,227,701. In the latter U.S. Pat. No. 4,227,701 a rupturable vent membrane is formed as an annular thinned portion on a downwardly sloping arm of the insulating sealing disk. The insulating sealing disk is slideably mounted on an elongated current collector running therethrough. As gas pressure within the cells builds up the center portion of the insulating sealing disk slides upwards towards the cell end cap, thereby stretching the membrane until it ruptures. U.S. Pat. No. 6,127,062 discloses an insulating sealing disk and an integrally formed rupturable membrane which is oriented vertically, that is, parallel to the cell's central longitudinal axis. When the gas pressure within the cell rises to a predetermined level the membrane ruptures thereby releasing the gas pressure to the external environment.
The rupturable membrane can be in the form of one or more islands of thin material within the insulating disk as shown in U.S. Pat. No. 5,589,293. Alternatively, the rupturable membrane can be in the form of a thin portion circumventing the cell' longitudinal axis as shown in U.S. Pat. No. 5,080,985. The circumventing thinned portion forming the rupturable membrane can be formed by grooves within the insulating disk as shown in U.S. Pat. No. 4,237,203. The rupturable membrane may also be a separate piece of polymeric film which is sandwiched between the metal support disk and the insulating disk and facing apertures therein as shown in patent application Publication U.S. 2002/0127470 A1. A pointed or other protruding member can be oriented above the rupturable membrane to assist in rupture of the membrane as shown in U.S. Pat. No. 3,314,824. When gas pressure within the cell becomes excessive, the membrane expands and ruptures upon contact with the pointed member, thereby allowing gas from within the cell to escape to the environment through apertures in the overlying terminal end cap.
A separate metal support disk, typically with convoluted surfaces as shown in U.S. Pat. Nos. 5,080,985 and 5,759,713, has been commonly included within the end cap assembly. The metal support disk withstands high radial compressive forces applied to the end cap assembly during crimping of the housing edge around the end cap assembly. The high radial compressive force assures that the seal between the end cap assembly can be maintained even if gas pressure within the cell builds up to a very high level, for example, between about 1100 and 1600 psig (7585×103 and 11032×103 pascal gage).
Alkaline cell end cap assemblies which do not include a metal support disk between the insulating sealing disk and terminal end cap are shown in U.S. Pat. Nos. 3,314,824; 3,617,386; 4,227,701; 4,237,203; 4,539,269 and 5,589,293. The end cap assemblies disclosed in these references generally have a high profile, that is, occupy significant percentage of the cell's internal volume or else do not reliably withstand gas pressure buildup of even a few hundred pounds per square inch gage (psig) buildup within the cell or else are formed of materials or have weak spots within the seal which are subject to chemical attack and corrosion by alkaline electrolyte.
Accordingly, it is desirable to have an alkaline cell end cap assembly which provides a tight seal for the cell during normal cell usage, yet has a low profile.
It is desirable that the end cap assembly have an insulating sealing disk with a rupturable venting mechanism therein.
It is desirable that the end cap assembly with rupturable venting mechanism be readily manufacturable and have as few components as possible to reduce cost, yet nevertheless enable gas venting at the desired pressure level.